1. Field of the Invention
The present invention relates to a digital communications transmission system having two terminals connected by means of a two-wire line, which terminals include a terminating set, a transmitter, a receiver and a calling device.
2. Description of the Prior Art
In analog technology, termination sets (bridge circuits) are used for the separation of the outgoing and incoming directions on two-wire lines, because the moderate stop-band attenuation which is dependent on the respective line impedance only causes an overhearing or superposition of its own speech signals. When the subscriber employs digital terminal devices, the moderate stop-band attenuation of the termination set can effect a disruption of the received signal by means of its own transmission signal. This is the case in both speech signal codes as well as in data devices.
The above difficulty can be countered either by means of the improvement of the termination set stop-band attenuation, or by means of the employment of segregated band systems. The former technique is effected according to the German allowed and published application No. 2,628,852 by means of automatic balancing of the simulation or, according to U.S. Pat. No. 3,993,867, by means of compensation of the transmission pulses in the receiver branch. The latter technique can be realized according to the German Pat. No. 2,040,401, either using time multiplex technology or with frequency division multiplex technology.
The processes for improving the termination set stopband attenuation have the disadvantage that a range significantly larger than 4 km cannot be achieved because the disruption of the weak reception signals by the outgoing transmission signals then becomes too great, even given improved termination set attenuation. Added thereto is the fact that, upon employment in the subscriber network, a reserve for the near-end crosstalk must be present.
Even in an arrangement functioning according to the time multiplex process, the range cannot be improved without further ado, because the consideration of the signal running time with increasing distance demands a greater compression of the bit sequences, which leads to higher transmission bit rates. Moreover, a transmission bit rate of, for example, 256 kbit/s has the inherent danger of coming into conflict with the interference beam protection conditions when the transmission system is employed on unshielded lines.
Moreover, as is known in the art, near-end crosstalk interference arises in multi-pair subscriber networks both in two-wire, as well as in four-wire operation.